Power from volcanic areas started in Italy 1903 (Larderello, tuscany) ca 800 MWe today
Conversion efficiency [%] 2 10 januari 2011 TNO Nieuwe huisstijl BRGM Geothermal power: Conversion Efficiency 25 20 15 binary 10 5 0 Tester et al. 0 100 200 300 400 Production temperature [C] flash
Natural fluid flow in magmatic areas Source: CNR-IGG-italy
Natural flow through the rocks Requires Pores (in sedimentary and volcanic deposits) Fractures and faults Less natural flow at depths larger than 2000m for pores
Depth [ km ] BRGM Temperature gradients in the upper crust Regional temperature variations Natural flow Direct heat power Conventional power EGS Supercritica; Temperature [ C]
Enhanced Geothermal Systems EU research project > 20 years 3 wells > 5 Km deep Comprehensive Fracturing programe 3MW el Power via ORC plant (www.soultz.net)
Power production Conversion 25%@300C, 10%@100C What is required flow rate Q [kg/s] for E=1000 MWe power for 300C resource geothermal fluid cooled down to 100C Heat capacity of geothermal fluid (Cp=4000 J kg -1 ) Q = E / (Cp DT 0.25 ) Q = 5000 kg/s Well is no more than 100kg/s 50 wells for 1000MWe, 20Mwe /well
Geothermal Energy From Exploration to Exploitation Adele Manzella CNR-IGG
Exploration and Investigation: the quest After 50 years of exploration a large amount of temperature data and significant knowledge of subsurface geology has been achieved. Several prospective areas for geothermal exploration can be outlined in Europe and many regions in the World. On what base have them been defined? Heat Heat flow flow map map and and power planned plants power in operation plants
Exploration and Investigation: the quest Apart direct shallow heat exchange of Geothermal Heat Pumps installations, subsurface heat is not used directly for power and heat production, but through a mass of water that exchanges and extracts the heat stored in the rocks. Water is really only the vector, but is a main element in our quest. The primary target of Exploratin and Investigation (E&I) are the socalled hydrothermal systems
Hydrothermal systems A geothermal system can be described schematically as "convecting water in the upper crust of the Earth, which, in a confined space, transfers heat from a heat source to a heat sink, usually the free surface".
Hydrothermal systems Elements of a hydrothermal geothermal system: a heat source a reservoir a fluid, which is the carrier that transfers the heat a recharge area
Hydrothermal systems The mechanism underlying geothermal systems is by and large governed by fluid convection. Convection occurs because of the heating and consequent thermal expansion of fluids in a gravity field. Model of a geothermal system. Curve 1 is the reference curve for the boiling point of pure water. Curve 2 shows the temperature profile along a typical circulation route from recharge at point A to discharge at point E (From White, 1973).
Hydrothermal systems A economically feasible geothermal reservoir should lie at depths that can be reached by drilling, possibly less than 4 km (accessibility requirement). A geothermal system must contain great volumes of fluid at high temperatures - a reservoir - that can be recharged with fluids that are heated by contact with the rock. productivity requirement For most uses, a well must penetrate permeable zones, usually fractures, that can support a high flow rate.
Hydrothermal systems When sufficient natural recharge to the hydrothermal system does not occur, which is often the case, a reinjection scheme is necessary to ensure production rates will be maintained. This would ensure the sustainability of the resource.
Hydrothermal systems The geological setting in which a geothermal reservoir is to be found can vary widely. The largest geothermal fields currently under exploitation occur in rocks that range from limestone to shale, volcanic and metamorphic rocks. Volcanic rocks are the most common single rock type in which reservoirs occur. Specific lithology do not define geothermal reservoirs
Hydrothermal systems High heat flow conditions plumes. rift zones, subduction zones and mantle Thick blankets of thermally insulating sediment cover basement rock that has a relatively normal heat flow lower grade Other sources of thermal anomaly: Large granitic rocks rich in radioisotopes Very rapid uplift of meteoric water heated by normal gradient
Let us define what we need Temperature as well as water amount are important for defining the feasibility of a geothermal resources for various, different uses. Example: power production Power is produced by the energy conversion of the thermal energy stored in the mass of water, into mechanical energy through a turbine, either directly (conventional flash technology) or undirectly (binary technology), and finally to electrical energy from the generator. 10 MW t (thermal) 1MW e (power)
Let us define what we need Example: power production (continues) To produce 1 MW e we need (rule of thumb) : 7-10 t/h of dry steam (over 250 C) 30-40 t/h of two-phase fluids at 200-250 C (flash technology) 400-600 t/h of water when using low enthalpy ORC binary cycles (120-160 C) The lower the temperature, the higher the amount of fluid required to produce a unit quantity of thermal (and electric) energy.
Goal: increase production In order to increase geothermal production, we need to increase the amount of fluid heated in the underground. This goal may be achieved by increasing heat exchange surface at depth, therefore, permeability within suitable geologic units: EGS (Enhanced or Engineered Geothermal systems)
The EGS concept is simple Fluid Permeability Hydrothermal Reservoir Temperature Enhanced Geothermal System (EGS)
The EGS concept is simple Goal: increase production For all intents and purposes, heat from the earth is inexhaustible. Water is not nearly as ubiquitous in the earth as heat. EGS concept covers specifically reservoirs at depth that must be engineered to improve hydraulic performance
Goal: increase production Numerous problems must be solved to reach the numerical goals and many unknowns need to be clarified: irregularities of the temperature field at depth favourable stress field conditions long-term effects, rock-water interaction possible thermal and hydraulic short circuiting EGS induced seismicity (during stimulation but also due to production) becomes a real issue; uniform connectivity throughout a planned reservoir cannot yet be engineered. scalability
Exploration and Investigation E&I techniques are used in all the geothermal project phases resource characterization geothermal gradients and heat flow, heat capacity, recoverable heat geological structure, including lithology and hydrogeology Tectonics induced seismicity potentials reservoir design and development fracture mapping and in-situ stress determination prediction of optimal re/injection and stimulation zones reservoir operation and management reservoir performance monitoring through the analysis of temporal variation of reservoir properties
Goals to be achieved by E&I To provide all necessary subsurface information to guarantee the best exploitation efficiency, the sustainability of the resource and the lowest possible environmental impact To reduce the mining risk by cutting the exploration cost and increasing the probability of success in identification of GS and EGS in prospective areas
Exploration&Investigation The objectives of geothermal E&I are: 1. To identify geothermal phenomena. 2. To ascertain that a useful geothermal production field exists. 3. To estimate the size of the resource. 4. To determine the type of geothermal field. 5. To locate productive zones. 6. To determine the heat content of the fluids that will be discharged by the wells in the geothermal field. 7. To compile a body of basic data against which the results of future monitoring can be viewed. 8. To determine the pre-exploitation values of environmentally sensitive parameters. 9. To acquire knowledge of any characteristics that might cause problems during field development.
Exploration&Investigation In order to understand the geothermal potential of a reservoir some relevant properties should be defined Geometry and type of fractures Geomechanical behaviour Reservoir Characterisation Fluid transport Temperature/Heat Flow State of stress
Exploration&Investigation Because of the high temperatures involved, both in the geothermal reservoir and in the source of the geothermal system, we can expect major changes to have taken place in the physical, chemical and geological characteristics of the rock, all of which can be used in the exploration project. Since heat diffuses alteration will be diffused too, and the rock volume in which anomalies in properties are to be expected will, therefore, generally be large.
Exploration&Investigation Goals to be achieved by Exploration&Investigation 1: before and during production Identify prospective reservoirs prior to drilling Define boundaries (lateral and vertical) (accessability) Identify drilling targets (productivity) Main permeability is driven by fracture and faults. wells $$$ avoid not economic wells
Exploration&Investigation Goals to be achieved by Exploration&Investigation 2: during and after production Continuously characterize the reservoir during energy extraction Follow the effect of production and fluid re-distribution, including the formation of steam or gas cap Characterize the rock fabric to define fluid flow paths within reservoir Predict fluid circulation during stimulation Track injected fluids Characterize formations during deep drilling and stimulation in order to predict reservoir performance/lifetime (effectiveness and sustainability)
Exploration&Investigation It is not possible to define a specific sequence of methodologies to be applied for the E&I of geothermal systems Choice is also related to cost Definition of a potential geothermal system Financial issues and economic feasibility of the project Evaluating social and environmental impact Regional screening Geology Geophysics Geochemistry Go Local/concessional screening No Go High resolution Geophysics and multidisciplinary approach Identification of a site Go No Go
A scale dependent approach Surface Geophysics (gravimetric, EM, Seismic), Airborne Lithosphere Strength Moho Depth Resource analysis Geology, Hydrogeology Heat Flow Stress Field Tomography Petrography, Petrophysics, Mineralogy Geochemistry, fluid geochmistry Hydraulic properties Borehole Geophysics (Acoustic Borehole Imaging, VSP,...) Continental Regional Local/Concessional Reservoir
Continental scale E&I Identification of potentially interesting regions of interest is based on: Task: Identify thermal field at great depths (>10km) from seismic tomography from thermal modeling Task: Identify Deformation regime of the crust from passive stretching models Extensional regimes can be of high interest Task: Identify Stress regime (neo-tectonics) from data cross-checking. Strike-slip regimes and extensional are the most interesting Task: Identify regions of interest
Continental scale E&I Seismic velocity anomalies from tomography (left), conversion of velocities to temperature, stress field, distribution of seismicity.
Regional scale E&I Heat flow analysis temperature gradient well data Seismic methods: focal mechanisms of earthquake smaller scale seismic events. Large-scale gravimetry: geometric trends of deep layers 2D/3D seismic profiles defining a geological model Electromagnetic prospection: apparent resistivity of rocks (link to geothermal reservoir not clearly established) Remote sensing identification of regional structures characterization of temperature fields Geochemistry identification of regional anomalies Task: Identify concessional areas
Concessional scale E&I Classical geophysical tools: 2D/3D seismic for geological mapping/identification of fault zones. Electromagnetic methods (MT-TEM-DC). Geothermal reservoirs > Low resistivity zone? Gravimetry. Geothermal reservoirs can have a gravimetric signature Resource potential analysis: integration of geological, hydrological, geochemical and geophysical data Estimation of energy recoverable from the reservoir. Cross-checking with infrastructure / areas of demand Economic viability of the system. Task: Identify reservoirs
Concessional scale E&I Key Parameters: Geometry of the aquifer Temperature at depth Hydraulic conductivity From Kohl et al., ENGINE Mid- Term conference
Reservoir scale E&I Well geophysics Vertical seismic profile, allows identification of structures at a distance from the well Borehole acoustic imaging and sonic log provides information about fractures crossing boreholes Borehole gravimetry can help defining conditions into the reservoir Gamma ray and resistivity logs provide information on the material surrounding the borehole Local stress determination stimulation strategy Conceptual model can be built, and assumptions verified with reservoir numerical model.
100 200 1.70 2.20 2.70 0.04 0.08 1 Depth (m) UBI Transit Time Facies Gamma Ray GAPI Bulk density g/cm3 Potassium % 3 Uranium ppm 10 25 40 45 60 75 75 100 125 6.25 9.25 12.25 200 300 0 45 90-0.5 Thorium ppm DT Comp µs/feet DT Shear µs/feet Caliper inch Cross Section Area cm2 Dip 0.5 Production log 8l/s 0 2 Injection log 18 l/s Permeable fracture Depth (m) BRGM Reservoir scale E&I Analogue models UBI Amplitude 3482 3483 3484 3485 3486 3487 3488 3489 3490 3491 3492 3493 3494 3495 3496 3497 3498 F-ma F-ma F-ma F-ma 3482 3483 3484 3485 3486 3487 3488 3489 3490 3491 3492 3493 3494 3495 3496 3497 3498
Geophysical exploration see separate presentations on: geochemistry geophysics
Case studies Geophysics is used for detecting and imaging overall geological features subsurface temperature fluid pathways stress field monitoring
OVERALL GEOLOGICAL FEATURES The best suited method in sedimentary and crystalline geological scenarios to extrapolate borehole information and to define and image the geological structure is the active seismic. Nowadays 3D seismic surveys are becoming standard in oil and mining industry, but are still far from being a must in geothermal exploration. However, due to the intrinsic complex 3D structure of geothermal areas, a successful 3D survey is the best way to retrieve a high resolution image of the subsurface geometry. 2D or 3D seismic must be calibrated by a comprehensive set of geophysical well logging data and petrophysical data. Expensive Cost reduction
OVERALL GEOLOGICAL FEATURES BASE OF NEOGENE BASE OF FLYSCH 3D Seismic data 3D well data Geophysical well data TOP OF METAM. BASAMENT TOP OF GRANITES STRUCTURAL INTERPRETATION From ENEL, ENGINE Worhshop1
OVERALL GEOLOGICAL FEATURES In volcanic rocks TDEM and MT have defined the main structure, driven mainly by alteration minerals From Karlsdottir, ENGINE Worhshop1
OVERALL GEOLOGICAL FEATURES Partially molten intrusives, representing the heat source in most of geothermal fields, at depths as shallow as 10 to 20 km produce thermally excited rocks which define high regional heat flow Demagnetised rocks confirm the existence of a hot rock mass in the crust Anomalously hot mass of rock delay the transit of the compressional (p) waves from earthquakes and reduce the amplitude of the shear (s) waves Density reduction due to partial melts may also be detected by gravity anomalies.
OVERALL GEOLOGICAL FEATURES 2/3D Modeling, properly balanced with experimental density data, pointed out deep low density bodies to be related to molten intrusions From ENEL, ENGINE Worhshop1 Low velocity bodies defined by teleseismic tomography and corresponding low resistivity bodies
OVERALL GEOLOGICAL FEATURES Resistivity decreases with increasing porosity and increasing saturation. Wave velocity is reduced by increasing porosity but shows different behaviour for different saturation, with an inverse relationship when saturation is high (100/85%) and a direct relationship when saturation is low, being constant for saturation of 15-85%. From Trappe, ENGINE Worhshop1 Thermal conductivity depends also on the porosity of the formation.
Depth (m) Heat flow (mw/m2) BRGM SUBSURFACE TEMPERATURE From Norden (left) and Bellani (below), ENGINE Worhshop1 600 Computed HF With proper care, heat flow and gradient data are able to define T distribution at depth 400 200 0 0 Observed HF Canneto 4 Colla 2 169 Carboli C bis -2000 346 286 322 238 250 326 Magnetic provides info regarding T (demagnetization at Curie T ) -4000 K horizon -6000 0 5000 10000 15000 20000 Horizontal distance (m)
SUBSURFACE TEMPERATURE Through a neuronet analysis of MT and T data, incorporating also geological information, electromagnetic data may be used as geothermometers. An example is shown for Bishkek site in Tien Shan (Spichak, ENGINE Worhshop1). Measured and modeled T distribution in wells. Solid line: measured T ; dashed line: modelled T based on T data only; modelled T based on T and MT data.
FLUID PATHWAY Many geophysical methods are able to map main lineaments and faults Inferred fault From Place, ENGINE Worhshop1 Oskooi et al., 2005 But this is not enough since there is still no direct evidence of fluid circulation
FLUID PATHWAY The correspondence between areas of low resistivity inside the resistive basement and geothermal reservoirs was very evident in the Mt. Amiata water-dominated system. When a fault defined by 2D reflection seimic corresponds to a low resistivity anomaly > water and/or clay Heat flow provide extra data Exploted geothermal field
FLUID PATHWAY Geophysical well logging by means of: Elastic/Acoustic and resistivity parameters Waveform analysis WSP (Well Seismic Profiling): VSP SWD 360 Hole Imaging From Dezayes, ENGINE Worhshop1 From ENEL, ENGINE Worhshop1 These data contrains seismic and MT, which are necessary for 3D extrapolation
FLUID PATHWAY When permeability concentrate in sub-horizontal layers an encouraging correlation was found between seismic reflections and fractures (red dots) through AVO analysis From ENEL, ENGINE Worhshop1
Fehler et al. 2001 BRGM FLUID PATHWAY Observing small mining produced seismic event has been called seismic monitoring. Events produced from fluid flow but also from internal subsidence have been successfully recorded and used to study fluid flow in time and space. Much larger events in reservoirs are generated during stimulation with artificial hydro-fracs. Monitoring the development of those fracs is usually called fracture monitoring. Soultz Hot Dry Rock a) Events from two injections and monitoring test b) Volume with events from both injections
FLUID PATHWAY By full wave 3D modelling of broadband seismological data it is possible to detect the formation of gas bubbles in the fluid due to pressure decrease. Definition of: Source location related with hydrothermal manifestations along known faults Geometry of fractures Gas/liquid ratio of the fluid From Manzella et al, ENGINE Launching Conference 500 m
FLUID PATHWAY Quantitative fracture prediction is made possible by modern reflection seismic concepts From Trappe, ENGINE Worhshop1 Normalized fracture density after cokriging
STRESS FIELD Passive seismology, active seismic and borehole geophysical logging provide information regarding regional and local stress. Induced fractures (vertical induced fractures, enéchelon fractures, mechanic breakout or thermal breakouts) and post-stimulated fractures could be interpreted and measured on borehole image logs in Soultz. Their geometrical relationship with the present-day stress field could be derived or computed. From Dezayes, ENGINE Worhshop1
MONITORING Gravity monitoring surveys are performed also to define the change in groundwater level and for subsidence monitoring. Fluid extraction from the ground which is not rapidly replaced causes an increase of pore pressure and hence of density. This effect may arrive at surface and produce a subsidence, whose rate depends on the recharge rate of fluid in the extraction area and the rocks interested by compaction.
MONITORING Geothermal exploration at a low enthalpy field using ERT. The low resistivity zone is coincident with a known fault where warm and saline fluids mix with surface and fresh water. An example of monitoring the effect on resistivity change when fresh water is pumped out from a well at the center of profile: the increase of salinity and temperature in the subsurface decreases the resistivity
MONITORING Phase change of pore fluid (boiling/condensing) in fractured rocks can result in resistivity changes that are more than an order of magnitude greater than those measured in intact rocks Production-induced changes in resistivity can provide valuable insights into the evolution of the host rock and resident fluids. No examples or applications found in literature Some examples from SP (electric field) showing interesting results: is it possible to use the same kind of information in MT? To be defined